Producing speech sounds requires coordination of air flow from the lungs with movements of the tongue, lips and jaw. Expiration passes through the vocal cords towards the oral and/or nasal cavity where it comes into contact with the tongue and lips. Oscillation of the vocal folds converts expiratory air into intermittent airflow pulses that result in voiced sounds. If the vocal folds are open, allowing air to pass unobstructed, they do not vibrate. Sounds produced this way are voiceless.
The pharyngeal, oral, and nasal cavities of the upper respiratory tract act as resonant chambers which transform the airstream into recognizable sounds with special linguistic functions whose main articulators are the tongue, jaw and lips. Together these generate patterned movements to alter the resonance characteristics of the supra-laryngeal airway. Coordination of all these enables us to perfectly pronounce phonemes, morphemes, syllables, words and sentences in any language.
This coordination is regulated by neural networks in the central nervous system that control both articulation (the sounds themselves) and phonological processes (patterns of sound). For example, certain neurons in a particular network control the tongue to move in a certain way into a certain position and likewise send signals to the lips and jaw to take specific forms and positions as needed.
When optimizing pronunciation or perfecting an accent, new neural networks must be established so as to produce new speech sounds properly. It is not enough to rely on previously developed native-language neural networks; an entirely new set of neural networks must, through mimicry and training, be created later in life, when language learning is more labored than in childhood.
Patients suffering from speech disorders or speech impairments also benefit from rebuilding of speech-related neural networks that either never developed sufficient due to congenital deficiencies, or are the result of brain injuries or diseases that impair coordination of the speech organs.